RNS) Nearly one in five Americans say they would not vote for a Mormon president, a percentage that has hardly budged since 1967, according to a new Gallup poll.

It is unclear how the anti-Mormon bias will affect Mitt Romney, the presumed GOP presidential nominee, Gallup said, since just 57 percent of Americans know that he is a member of the Church of Jesus Christ of Latter-day Saints.

"This suggests the possibility that as Romney's faith becomes better known this summer and fall, it could become more of a negative factor," Gallup writes, "given that those who resist the idea of a Mormon president will in theory become more likely to realize that Romney is a Mormon as the campaign unfolds."

Still, Gallup noted that John F. Kennedy won the presidency in 1960 despite the 21 percent of Americans who said they would not vote for a Catholic president.

Separate Gallup polls show the former Massachusetts governor essentially tied with President Obama.

This year, nearly 8 in 10 Catholics, Protestants and religiously unaffiliated Americans said they would vote for a qualified Mormon candidate, with little statistical difference between the groups.

Nearly a quarter of Americans with a high school education or less said they would not vote for a Mormon; that number decreases to just 7 percent among those with postgraduate degrees.

Nine in 10 Republicans and 79 percent of independents said they would vote for a Mormon; just 72 percent of Democrats agreed.

Gallup began asking the Mormon question in 1967 when former Michigan Gov. George Romney, Mitt Romney's father, was a top candidate for the GOP nomination. That year,19 percent said they would not vote for a Mormon presidential candidate.

This year, 18 percent said they would not vote for a qualified Mormon candidate, down from 22 percent in 2011.

The anti-Mormon bias remains remarkably consistent, according to Gallup, considering that resistance to candidates who are black, Jewish or a woman has declined markedly since 1967.

Anti-Mormon sentiment tends to rise slightly when when Mormons are running for president, Gallup noted, with the all-time high of 24 percent coming during Romney's first presidential campaign in 2007.

The Gallup poll is based on telephone interviews conducted June 7-10 with a random sample of 1,004 adults. The poll has a margin of error of plus or minus 4 percentage points.